1. Field of the Invention
The present invention relates to a disc memory apparatus and, more particularly, to a servo signal recorded in a servo zone which is provided on a magnetic disc to detect a position.
2. Description of the Prior Art
As one of the magnetic discs, there has been known a disc having a servo zone in which information to detect a position has prerecorded on a unit basis of a predetermined length of a track, for instance, on a sector unit basis. FIG. 1 shows an example of a servo zone of a conventional magnetic disc. An alternate long and short dash line indicates a track center of each of the Nth and (N+1)th tracks which are formed at a track pitch Tp. Tw denotes a track width of the magnetic head for recording/reproducing. Since it is impossible to completely eliminate a tracking error, Tp and Tw are set such that (Tp&gt;Tw) and a guard band is provided between adjacent tracks. In general, Tp is selected to about (Tp=2/3 Tw).
A servo header 41 indicative of the start position of the servo zone is recorded at the head of the servo zone Sv. The servo header 41 denotes special data which can be distinguished from the digital data which is recorded/reproduced. An address region 43 in which an address AD of a track unit is recorded is provided after the servo header 41 Detection signals 45A and 45B to detect the positions are provided after the address region 43. The detection signals 45A and 45B are recorded as a diced pattern so as to be located on the inner and outer peripheral sides with respect to each center of tracks, respectively. On the other hand, the detection signals 45A and 45B are recorded at the position which are deviated in the track direction. Therefore, for instance, the levels of the respective reproduction signals can be independently detected at the timings based on the clocks synchronized with the reproduction signal of the servo header 41. The detection signals 45A and 45B are the signals in which pulse signals of a predetermined frequency, what is called, bursts are recorded.
In the above servo zone SV, the address AD regenerated from the address region denotes the coarse servo information (referred to as address information) of the position information. The fine servo information is formed from the reproduction signal of each of the detection signals 45A and 45B. That is, as shown in FIG. 2, the address AD is an integer value which changes step by step every track. The fine servo information SF denotes a value which changes like a sawtooth wave with a width of one track pitch Tp. Therefore, by adding the value of the address AD and the fine servo information SF, the position information (AD+SF) which continuously changes in the radial direction of the disc is formed. The subtraction is executed between the level of the reproduction signal from the detection signal 45A and the level of the reproduction signal from the detection signal 45B and the result of the subtraction is normalized as will be explained hereinlater, so that the fine servo information SF is formed.
The fine servo information SF is used to accurately position the magnetic head onto the track. On the other hand, in the seeking operation to access the target track, the position information (AD SF) is differentiated and the speed of the magnetic head is detected. By using the detected speed, there is executed a control to move the magnetic head in accordance with a desired speed profile.
With respect to the fine servo information SF, the level difference between the reproduction signals of the detection signals 45A and 45B cannot be directly used. This is because the levels of the reproduction signals of the detection signals 45A and 45B fluctuate due to the difference between the recording and reproducing capabilities of the magnetic head and the magnetic disc, the difference of the linear speeds between the inner rim side and the outer rim side, the difference between the flying heights, and the like. Hitherto, to solve such a problem, a sum signal of the reproduction signals of the detection signals 45A and 45B is obtained, thereby executing the normalization to divide the difference signal by the sum signal.
The conventional normalization will be described with reference to FIG. 3. With respect to the radial direction of a certain track, for instance, the Nth track, it is assumed that there is a deviation of x between the center of the magnetic head and the track center (refer to FIG. 1). When x is set to an axis of abscissa and the relative level (the maximum value is set to 1) of the reproduction signals is expressed as y, reproduction outputs SA and SB of the detection signals 45A and 45B change for x as shown in FIG. 3A. When the track center is set to 0, the reproduction output SA is set to 0 in a range from x=-1/2 Tp to x=1/2 (Tp-Tw) and is set to the maximum value at x=1/2 Tp. On the other hand, the reproduction output SB is set to 0 in a range from x=1/4 Tp to x=1/4 (Tw -Tp) and is set to the maximum value at x=-1/2 Tp.
The difference (SA-SB) between the reproduction outputs is as shown in FIG. 3B and is set to 0 at x=0. The sum (SA+SB) of the reproduction outputs is as shown by a solid line in FIG. 3C. Therefore, the normalized fine servo information SF (=(SA-SB)/(SA+SB)) shown by a broken line in FIG. 3C is derived.
The reproduced outputs SA and SB shown in FIG. 3A are set to 0 in a range from .+-.1/2 (Tp-Tw) to .+-.1/4 Tp because of the relation of (Tp&gt;Tw), so that the difference output shown in FIG. 3B becomes like an S-shape. Further, the sum signal shown by a solid line in FIG. 3C has a level which changes with x. From the above relation, the fine servo information SF obtained by the normalization is such that the linearity is worse than that of the original difference signal and in a range from .+-.1/4 (Tp-Tw) to .+-.1/4 Tp the level is constant with respect to the change of x, that is, the sensitivity is set to zero. Such a fine servo information SF of bad linearity prevents accurate tracking to the target track and causes a problem such that an accurate speed detection cannot be performed from the position information.
On the other hand, in the seeking operation, since the magnetic head moves while obliquely transversing a plurality of tracks, it is necessary to accurately obtain the position information at any position on the disc. When the magnetic head is located between adjacent tracks, the address regions 43 of the respective tracks are simultaneously reproduced, so that there occurs a problem such that the correct address cannot be read. To solve the above problem, as disclosed in U.S. Pat. No. 4,032,984, there has been proposed a method whereby the address AD is set to a code of a gray code and the neighboring property of the gray code is used. However, in the case of the gray code, since it is necessary to preserve the neighboring property, there is a subject such that the digital modulating system which can be used is limited.